Page 76 - Theory and Design of Air Cushion Craft

60 Air cushion theory

Considering that the jet nozzle Re on the big skirt test rig is in the turbulent region
and the effect of H }/H 2 to p c can be neglected, then the relative cushion pressure can
be written as follows:
= f,(h/t,S/B b,x/B) (2.20)
p c
In order to save time for test and analysis and also considering that the foregoing for-
mula should be processed by three-dimensional regression analysis, the test projects
can be arranged as shown in Table 2.4 and also taking the p as the function of rela-
= f(hlt). Then the correction with respect to and
tive hovering height, namely p c S/B b
x/B can also be considered. The same method can be used with the other characteris-
tics of hovercraft such as cushion flow rate coefficient, cushion power coefficient and
skirt bag pressure coefficient to derive these as well. These coefficients can be written
as follows:
C =
A = A /A
5
m = ml(p.j) cf = f m(h/t)
n = (Npl' 5 )//>; 5 = / p(/i/0 (2.21)
is the cushion pressure coefficient, the
where p c namely the relative cushion pressure, p t
bag to cushion pressure ratio coefficient, m the cushion flow rate coefficient, h the
cushion power consumption coefficient, m the mass flow rate per unit area of air
2
4
curtain, m = Qpjlfi, l } the peripheral length (m), p a the air density (Ns /m ), Q the
cushion volumetric flow rate (m /s) and N the power consumption of hovercraft
per unit area of air curtain (Nm/s per m'). All of the coefficients mentioned above are
dimensionless.

Analysis of streamlines from tests
Due to the large size of the skirt test rig, the streamlines may be plotted (Figs 2.10 and
2.1 1) for two hovering conditions, one for large hovering height (hit < 1) and one for
small hovering height (hit = 0.15).
The length and thickness of the enclosing wall for the jet (the total pressure of flow
will drop dramatically outside such thickness so Pitot and total pressure measure-
ments can be used to survey it), the total pressure and the separation point B can be
measured. The streamline chart for large/small hovering height was obtained as
shown in Figs 2.10 and 2.11 respectively. It is found that the following physical phe-
nomena are observed:
• The length and effect of enclosure for a jet with definite angle can be found by test-
ing, although in the case of the bag and finger skirt, in which the flow will diffuse
suddenly at the location of holes, it will also be related to the location of the holes
and hovering height.
• Owing to the effect of the enclosing wall on the jet, the air flow at the segment tip
possesses the function of sealing the air cushion, especially in the case of large air
clearance (Fig. 2.10) and the separation point B appears very close to the ground.
Its position inside or outside the skirt tip line is dependent on the cushion pressure